The present invention relates generally to the field of magnetic resonance imaging systems such as though used in medical imaging applications. More particularly, the invention relates to a technique for correcting image artifacts resulting from instabilities in a magnet system by use of phase, position and amplitude information acquired from a navigator echo integrated into a pulse sequence description of an imaging examination.
Magnetic resonance imaging systems have become an extremely useful tool for medical applications, permitting non-invasive diagnostics of a range of anatomies and tissues. In general, MRI systems produce excitations in gyromagnetic material within a selected slice of a patient, and then detect emissions from the gyromagnetic material for reconstruction of a useful image. In general, a main or primary magnetic field is generated by a strong magnet surrounding a patient bore or other region in which the anatomy to be imaged is positioned. Gradient coils produce gradient magnetic fields which are properly oriented to select a slice of tissue to be imaged, to phase encode specific locations or volume elements (voxels) within the tissue, and frequency encode the voxels. Radio frequency pulses excite the gyromagnetic material, and a receiver coil detects the resulting emissions. Following conditioning of the resulting signals, and two-dimensional fast Fourier transformation, the useful image may be reconstructed wherein individual picture elements or pixels correspond to the voxels of the selected slice.
It has been found that, in MRI systems, instabilities within a magnet system can produce time-dependent variations in the main magnetic field. Again, the main magnetic field is produced by a fairly strong magnet, the field of which is oriented horizontally (such as in most conventional scanners) or vertically (such as in xe2x80x9copenxe2x80x9d scanners). In addition to the variations in the main magnetic field, such instabilities may also result in time-dependent variations in the spatially linear fields produced by the gradient coils. Such variations cause the gyromagnetic material to be imaged to be excited or encoded in a manner different from that predicted in establishing the pulse sequence description used to drive the coils and produce the magnetic fields. As a result, artifacts may be visible in the reconstructed image which may adversely affect the image clarity, and reduce the utility of the image.
There is a need, therefore, for an improved technique for correcting or compensating for instabilities in an MRI magnet system which can improve image quality. In particular, there is a present need for a straight forward system which can be implemented in a wide variety of systems to detect and compensate for magnet system instabilities to eliminate or substantially reduce the occurrence of imaging artifacts.
The present invention provides a correction or compensation technique for MRI systems designed to respond to these needs. The technique may be applied to new and existing systems, and may be implemented through software used to define a pulse sequence description on which the gradient and radio frequency pulses are based. While variants of the technique may be employed to correct for deviations in spatially higher order terms in the magnet system, the technique is particularly well suited to detection and correction of perturbations in the magnet system performance due to variations in zeroth order and spatially linear fields produced by the gradient coils and environmental factors, such as support structures, floors, and so forth. The technique allows for characterization of various magnet system instability effects, and correction of image data based upon these characterizations.
In accordance with certain aspects of the technique, phase, position and amplitude information is collected from a navigator echo acquired along with image data in an imaging sequence. The navigator echo characterizes the imaging effect of time-dependent field changes. In general, the navigator echo is an echo signal which is acquired without application of phase encode gradients or with the effect of phase encode gradients rewound before data acquisition. The placement of the navigator echo in the pulse sequence may be such that the echo is in close proximity in time to the regular image echo, such that the characterization is accurate and complete, permitting variations in the magnet system performance to be accurately corrected in the resulting image data. The resulting corrections may account for the effects of the instabilities on parameters of the acquired data, such as amplitude, zeroth order phase shifts, and first order phase shifts. Moreover, the characterizations may correct for position shifts, and combinations of these effects in the resulting image data.